Lo

A minimal logger for Go.

Lo is a fork and modification to the official Go log package (https://golang.org/pkg/log/). It's retained only the Printf function for writing messages and has added three levels of logging (none, info, and debug). Lo's been influenced by a number of articles, discussion and personal experience.

Things to note:

With the log level set to lo.LevelNone, lo isn't nop. Using a single Printf function this isn't possible but it is minimal.
With the log level set to info, debug messages aren't nop for the same reason above.
INFO and DEBUG are prefixed to the format string for easier log passing.
Lo only implements Printf as this is the minimal interface a logger needs. This makes switching out this logger for something else in the event you need different features easy. (inspired from discussion)
Lo only has NONE, INFO and DEBUG levels. From experience that's all I use. (inspired by Dave Cheney)
The package level logger has been removed so you need to create a lo logger and pass that around your app. (inspired by Peter Bourgon)

Installation

Using dep for dependency management (https://github.com/golang/dep):

$ dep ensure github.com/dyson/lo

Using go get:

$ go get github.com/dyson/lo

Usage

Basic

package main

import (
	"fmt"
	"os"

	"github.com/dyson/lo"
)

func main() {
	logger := lo.New(os.Stdout, "", lo.LstdFlags)

	fmt.Println("show only info messages by default:")
	logger.Printf("info message")
	logger.Printf("debug: debug message with a space")
	logger.Printf("debug:debug message without a space")

	fmt.Println("\nshow info and debug messages:")
	logger.SetLevel(lo.LevelDebug)
	logger.Printf("info message")
	logger.Printf("debug: debug message with a space")
	logger.Printf("debug:debug message without a space")

	fmt.Println("\ndisabled logging:")
	logger.SetLevel(lo.LevelNone)
	logger.Printf("info message")
	logger.Printf("debug: debug message with a space")
	logger.Printf("debug:debug message without a space")
}

Running example:

$ go run main.go
go run main.go
show only info messages by default:
2017/08/02 11:15:07 INFO info message

show info and debug messages:
2017/08/02 11:15:07 INFO info message
2017/08/02 11:15:07 DEBUG debug message with a space
2017/08/02 11:15:07 DEBUG debug message without a space

disabled logging:

When passing around the logger you should accept an interface: (inspired by Dave Cheney)

type logger interface {
	Printf(string, ...interface{})
}

Nop

Using the logger interface above you can easily implement a nop logger if you want to disable logging:

package main

import (
	"fmt"
	"os"

	"github.com/dyson/lo"
)

type logger interface {
	Printf(string, ...interface{})
}

type nopLogger struct{}

func (l *nopLogger) Printf(format string, v ...interface{}) {}

func main() {
	var logger logger
	disableLogging := true // set from ENV var or command line flag, etc
	if disableLogging {
		logger = &nopLogger{}
	} else {
		logger = lo.New(os.Stdout, "", lo.LstdFlags)
	}

	fmt.Println("nop logger:")
	logger.Printf("info message")
	logger.Printf("debug: debug message")

	disableLogging = false
	if disableLogging {
		logger = &nopLogger{}
	} else {
		logger = lo.New(os.Stdout, "", lo.LstdFlags)
	}

	fmt.Println("\nlo logger:")
	logger.Printf("info message")
	logger.Printf("debug: debug message")

}

Running example:

$ go run main.go
nop logger:

lo logger:
2017/08/02 13:39:14 INFO info message

License

See LICENSE file.